GLP-1 BMI Calculator

What BMI is and what it isn't

BMI is a simple ratio of body weight to height squared (kg/m²) that the World Health Organization uses to classify adults into six categories: underweight, normal weight, overweight, and three classes of obesity [1, 2]. It's a population-level screening tool, not a diagnosis. BMI does not distinguish between fat mass and lean mass, doesn't account for body fat distribution, and can over- or under-classify athletes, older adults, and people with certain body compositions.

For GLP-1 prescribing decisions, however, BMI is the specific clinical criterion the FDA uses to define eligibility, so even though it's an imperfect tool, it's the tool that determines whether you can get a prescription [3, 4].

FDA eligibility for Wegovy and Zepbound

Both Wegovy (semaglutide 2.4 mg) and Zepbound (tirzepatide 15 mg) have the same FDA-approved BMI eligibility criterion for chronic weight management [3, 4]:

• BMI ≥ 30 (any class of obesity), OR
• BMI ≥ 27 WITH at least one weight-related comorbidity such as type 2 diabetes, hypertension, sleep apnea, dyslipidemia, or established cardiovascular disease.

Patients below BMI 27 are generally not eligible under the FDA-approved labeling. Off-label prescribing exists but is uncommon for weight management at lower BMI ranges.

What the GLP-1 overlay shows

The trial outcome cards below the BMI result show the predicted BMI and category you'd land in if you achieved the published mean weight loss from each phase 3 trial:

• STEP-1 [5] — semaglutide 2.4 mg, 14.9% mean body weight loss at 68 weeks
• SURMOUNT-1 [6] — tirzepatide 15 mg, 20.9% mean body weight loss at 72 weeks
• ATTAIN-1 [7] — orforglipron (Foundayo), 12.4% mean body weight loss at 72 weeks

The math is direct: your starting BMI multiplied by (1 - mean loss percentage / 100). Real outcomes vary substantially by individual — see our weight loss calculator for the full conservative/optimistic range and the week-by-week milestones.

Important caveats

• BMI is a population tool, not a diagnostic. Two people with the same BMI can have very different body compositions and very different cardiometabolic risk profiles.
• FDA eligibility is the floor, not the ceiling. Insurance coverage decisions often add additional criteria (lifestyle program documentation, BMI ≥ 35, specific comorbidities) on top of the FDA approval. See our insurance employer checker.
• Individual variation is large. The trial mean values are averages across thousands of patients. About 10-15% of trial participants lost less than 5% even at the maintenance dose; about 10% lost considerably more. See our plateau and non-responder guide for the response distribution.
• Body composition matters more than total weight. Up to 40% of weight lost on a GLP-1 can be lean mass if protein intake and resistance training are inadequate. See our muscle mass deep-dive.
• Discontinuation rebound. The STEP-4 and SURMOUNT-4 trials showed that patients who stopped GLP-1 therapy regained roughly 67% of the lost weight within a year. See our tapering guide.

Important disclaimer

This calculator is for educational purposes only and does not constitute medical advice. BMI is a screening tool, not a diagnosis, and FDA eligibility is one of many factors a prescriber considers when deciding whether to start GLP-1 therapy. Individual outcomes vary substantially. Weight Loss Rankings does not provide medical advice, diagnosis, or treatment recommendations.

Related tools and research

• GLP-1 weight loss calculator — week-by-week trial-data-backed predictions
• GLP-1 dose plotter — pharmacokinetic buildup curves for each titration step
• GLP-1 unit converter — mg ↔ insulin syringe units for compounded vials
• Tirzepatide vs semaglutide head-to-head — full effect-size comparison
• Foundayo (orforglipron) approval deep-dive — the new oral option